1. Field Of The Invention
This invention relates to relief valves for providing pressure relief in a pumping system during treatment of wells, and more particularly, to a relief valve having a double poppet, one poppet portion for orificing and another poppet portion for sealing.
2. Description Of The Prior Art
In the hydraulic fracturing of oil and gas wells, fluids are used to which proppants have been added. The recent trend has been to use higher and higher proppant concentrations to increase production as much as possible. As the proppant concentration increases, so do the difficulties associated with pumping the fluid. One problem is that of undesirable pressures which may be a result of a screenout, mechanical failure or human error. These undesirable pressures can cause damage to the well formation, the well casing and tubing therein, and the pumping equipment, in addition to being a possible hazard to personnel.
A screenout occurs when the fracture in the formation will physically not accept any more proppant. This results in a pressure rise measurable at the surface. This increase in pressure, or pressure "spike", can occur over just a few seconds, and increases in the range of 600 to 4000 psi are not unusual. These pressure spikes occur too rapidly for operators to react. Therefore, a pressure relief device is required in the pumping system.
Other undesirable increases in pressure may occur due to a malfunctioning pressure gauge. Also, valves in the manifolding or on the well head may be improperly in a closed position or may be closed during the pumping operation. Again, a pressure relief device must be provided for the system. These pressure related problems can also occur when merely pressure testing lines.
Pressure relief valves have been used in such pumping systems for some time and are well known. Typically, the relief valve is connected to the discharge line of the pump, and the relief valve consists of a housing structure, a seat, a poppet and some means of applying force, such as pneumatic or hydraulic pressure source. The poppet generally performs two functions. The first is a sealing function, and the second is an orificing function. Such a previously known relief valve, which is described in more detail herein, has a problem in that after opening, it will not reseal. This is due to the fact that when throttling the fluid during the orificing function of the valve, the proppants in the fluid, which are extremely abrasive, erode the sealing components. Typically, this erosion causes the valve to leak after only one operation. Such erosion can occur over a time span of just a few seconds. Obviously, if the valve will not reseal, it will leak, and this leakage is undesirable because it creates the potential for well flowback which can result in loss of fracturing fluid and is potentially hazardous.
The result is that the valve should be repaired after every operation. This is not always done, and at times, the job is continued at a slower rate with the relief valve blanked off because of the leakage. This is obviously undesirable because this last pumping stage is done without any relief valve in the system.
Previous attempts to solve this erosion problem with relief valves have been mainly concentrated on improvements in valve materials, such as using hardened materials and the like. Such hardened materials are generally expensive, and even so, the use of such materials has had little success because of the kind of fluids used in oil and well fracturing operations.
The relief valve of the present invention has a double poppet means with separate orificing and sealing poppet means. When the valve is in the open position, the orificing means performs the throttling or orificing function where high fluid flow rates are present, and the sealing poppet means is kept in a zone of low pressure drop and low fluid velocity. While open, the orificing poppet means and the adjacent orificing surface sustain the erosion, and the sealing poppet means sustains relatively little wear. This concept, used in conjunction with wear-resistant materials, provides extended life for the sealing components. The major advantage is that the valve may be operated several times without requiring repair.